The invention is based on a priority application EP05290616.1 which is hereby incorporated by reference.
The present invention relates to a bi-directional communication interface and to a method for testing it for defects.
A preferred but not exclusive application of the present invention is a DSL (digital subscriber line) modem and a method for testing it. The modem may be part of a digital subscriber line access multiplexer (DSLAM) or of a subscriber's equipment.
Alleged or real failures of DSLAMs operating in the field contribute significantly to the operating cost of a telecommunication network. Since the DSLAMs have to be installed closed to a subscriber's premises, it is not possible to group many of them at a same location. Whenever one of these DSLAMs fails or seems to fail, service personnel has to be sent out to check the DSLAM, involving considerable costs for the operator of the network.
Failures may occur in DSLAMs for various reasons. A frequent cause of failure are their power supply devices. A failure of these is easily detected from a management station of a network to which the DSLAMs are connected, since they cause the complete circuit board of the DSLAM to fail. Other failures which are not so easily detected are single component damages. These occur quite frequently with digital or analog ASICS, since these are active silicon components with a high degree of integration, the transistors of which are sensitive to overvoltages, electromagnetic interference, etc.
Another important cause of failures are line drivers and components associated to these, since they have a high power dissipation, and because they are directly connected to the telephone line (subscriber line), where overvoltages due to lightning may occur.
Another frequent cause of communication problems between DSLAM and subscriber modems are errors in the configuration of ATM and IP layers of the various network components such as switches, routers, and broadband access servers to which the DSLAM is connected on the network side. Such errors can make the DSLAM appear defective, while it is in fact only incorrectly controlled.
Similar communication problems may be caused by the subscriber's modem, if settings in the subscriber's equipment are incorrect.
In particular in the latter cases, it is quite frequent that repair staff is sent to a DSLAM because it seems defective, but in the end, the effort is in vain, because the reason for a failure is somewhere else.
Network operators are of course interested in keeping the number of such visits as low as possible. In order to meet this demand, DSLAMs are severely tested before delivery.
In a conventional pre-delivery test, all XDSL ports of a DSLAM card are connected to a reference impedance of 100 ohms. Then a set of automatic tests carried out by a processor of the DSLAM allows for detection of hardware problems in the DSLAM. As this type of test needs a known reference impedance, it cannot be carried out in the field, where telephone lines are connected to the DSLAM ports, the impedance of which is not known exactly, and on which there may be noise signals of various origins.
A paper by Acterna, LLC, Germantown, Md. entitled “Verification of ADSL Modem Interfaces as per ANSI T1.413 and ITU-T G.992.1” describes a method for testing an ADSL transmitter in which the transmitter is connected to an ADSL line simulator. In the spectrum of DMT carriers that form a conventional ADSL signal, a gap is formed by suppressing one or more carrier frequencies, so that intermodulation noise generated at the suppressed frequency can be observed without background, and a signal-noise ratio at the suppressed frequency or frequencies is obtained from measured power levels of said intermodulation noise and of an unsuppressed DMT carrier to the left or the right of the gap.
The so-called boot self-test, which is conventionally performed by a DSLAM when powered up, allows to detect some hardware problems, mainly in the digital circuitry of the DSLAM. Problems of the analog front-end of the DSLAM and of the subscriber line are not detected. Moreover, the power-up self-test cannot detect problems that arise during operation, because in order to repeat the self-test, the DSLAM would have to be re-booted, which would imply an interruption of service for all users connected to it, which cannot be tolerated.
Another conventional testing method which is useful for testing the communication between the DSLAM and a subscriber's modem implies the use of two protocol simulator circuits. For carrying out this test, the connection between the DSLAM and the subscriber's modem is interrupted using relays placed between the DSLAM and the telephone line, and the DSLAM is connected to the protocol simulator which simulates the subscriber's modem, and the subscriber's modem is connected to a DSLAM simulator. If it turns out in the test that the subscriber's modem cannot communicate with the simulator associated to it, but the DSLAM can, is shown that the DSLAM is operative, and that the defect must be at the subscriber's side. Such a test can be carried out without sending staff to the DSLAM, if the two simulators and remote-controlled relays for establishing the required connections are present at the DSLAM. The use of this technology therefore requires considerable investment.